High frame rate ultrasonic diagnostic imaging systems with motion artifact reduction

ABSTRACT

An ultrasonic diagnostic imaging system and method are provided for producing r.f. interpolated image lines with reduced susceptibility to motion artifacts. A multiline beamformer receives multiple scanlines in response to each transmitted beam. Image lines are produced by r.f. interpolation of these multiline scanlines in a temporally consistent manner. In an illustrated embodiment, each image line is produced by tle interpolation of scanlines produced in response to at least two transmitted beams. In a preferred embodiment the interpolated image lines are produced by a [1 2 1] lateral filter.

This invention relates to ultrasonic diagnostic imaging systems and, inparticular, to ultrasonic diagnostic imaging systems capable ofproducing high frame rate ultrasonic images with reduced motionartifacts.

U.S. Pat. No. 5,706,819 describes a signal processing technique whichseparates fundamental and harmonic signal components in receivedultrasonic echo signals. This technique, known in ultrasound as “pulseinversion,” is a two pulse technique in which two pulses of opposingpolarity (phase) are successively transmitted to the same location inthe body. Echoes are received following each transmission in whichfundamental signal components are out of phase due to the opposingpolarity of the transmit pulses, but the higher order harmonic signalcomponents, being quadratic in nature, are not. Summing the two echoeswill cancel the opposing fundamental components and reinforce theharmonic components, leaving a cleanly separated harmonic signal withoutthe need for conventional filters. Subtracting the two echoes will havethe opposite result, canceling the harmonic signal components andreinforcing the fundamental (linear) signal components. In a similarmanner, subtraction leaves a cleanly separated fundamental echo signal.

Pulse inversion is a two pulse technique, however, meaning it isnecessary to scan each acoustic line twice in order to form a singleimage. This means that the time required to acquire all of the scanlinesof an image is approximately doubled as compared to conventional singlepulse imaging. The time to acquire all of the scanlines of an imageframe determines the frame rate of display, which will approximatelyhalve with a two pulse technique. It is desirable to have as high aframe rate as possible so that realtime imaging is produced which showstissue motion smoothly and with little interframe discontinuity as ascanhead is moved when surveying a patient's anatomy.

In a concurrently filed application it is shown how pulse inversionharmonic imaging can be carried out at a high frame rate of display anda high line density. In one embodiment of that inventive techniquetransmit pulses of opposing polarity (phase) are transmitted alongtransmit scanlines at adjacent positions in the image field and multiplescanlines are received in response to each transmitted beam. Receivedscanlines from opposite polarity pulses are combined to produce harmonicimages at a high frame rate of display. By combining received scanlinesin a temporally consistent manner, motion artifacts are reduced. In thepresent invention, this principle is applied to reduce motion artifactswhen performing r.f. interpolation of multiline scanlines. The inventivesystem employs a multiline beamformer which receives and forms multiplereceived scanlines in response to a single beam transmission.Interpolated scanline image data is produced by interpolating temporallydifferent scanline data for each image line. The image data is then usedto form an ultrasonic image. The scintillation effect of motion iseliminated by the use of temporally different scanline data to form eachimage line.

In the drawings:

FIG. 1 illustrates a pulse inversion scanning technique of the presentinvention which produces a high line density image;

FIG. 2 illustrates a pulse inversion scanning technique of the presentinvention which produces ultrasonic images at a high frame rate ofdisplay;

FIG. 3 illustrates a variation of the pulse inversion scanning techniqueof FIG. 2;

FIGS. 3A-3D illustrate the development and removal of artifactsassociated with the scanning technique of FIG. 3;

FIG. 4 illustrates a pulse inversion scanning technique of the presentinvention using multiline scanline reception;

FIG. 5 illustrates in block diagram form the receiver of an ultrasonicdiagnostic imaging system for processing signals in accordance with theinventive technique of FIGS. 1-3;

FIG. 6 illustrates in block diagram form the receiver of an ultrasonicdiagnostic imaging system which employs a [1 2 1] filter function;

FIG. 7 illustrates in block diagram form the receiver of an ultrasonicdiagnostic imaging system for producing both separated fundamental andharmonic signals which are blended into a common image;

FIG. 8 illustrates in block diagram form the receiver of an ultrasonicdiagnostic imaging system using multiline reception for producing pulseinversion separated harmonic signals in accordance with the principlesof the present invention;

FIG. 9 illustrates application of the principles of the presentinvention in single pulse imaging to reduce motion artifacts; and

FIG. 10 illustrates in block diagram form an ultrasonic diagnosticimaging system which produces images with reduced motion artifacts inaccordance with the inventive technique of FIG. 9.

Referring first to FIG. 1, a pulse inversion scanning technique of thepresent invention is shown. In FIGS. 1-4 and 9 the vectors of an imagefield (the region of the body which is being scanned) along whichultrasonic waves are transmitted and received are represented by arrows.These scanline arrows are shown in a relative spatial orientation inrelation to the elements e of a linear array transducer 10 whichtransmits and receives the scanlines. The scanlines are depicted in alinear format, however they may also be transmitted and received in asector or steered linear format as is known in the art. Scanline arrowspointing down in these drawings depict transmit scanlines and scanlinearrows pointing up depict received scanlines. Straight lines depictimage lines in their relative positions in an image.

In pulse inversion scanning in accordance with my aforementioned patent,two pulses of opposing phase or polarity are transmitted along eachscanline direction, as represented by the paired transmit scanlines, thefirst pair of which is shown by transmit scanlines 1+ and 2−, with the“+” indicating a positive polarity transmit pulse and the “−” indicatinga negative polarity transmit pulse. The relative phase opposition of thetwo transmit pulses or waveforms is preferably 180°; a lesser differenceyields less than complete separation of linear and harmonic signalcomponents when the resulting echoes are combined.

When pulse inversion is performed as shown in my patent, transmitscanlines 1+ and 2− yield received scanlines R₁₊ and R²⁻, respectively,with the number indicating the corresponding transmit scanline. Transmitscanlines 3+ and 4− yield received scanlines R₃₊ and R⁴⁻, and so forthalong the array. Echoes along each received scanline are then summed oradded on a common depth (z) basis to cancel fundamental signalcomponents from tissue or contrast agents and leave only the harmonicsignal components of the received echoes. Thus, the summation ofreceived scanlines R₁₊ and R²⁻ produces an image line L1 of harmonicsignals, the summation of received scanlines R₃₊ and R⁴⁻, produces animage line L3 of harmonic signals, and so forth.

In accordance with the principles of the invention of the concurrentlyfiled application, adjacent scanlines received from transmit scanlinesof opposite polarity are summed to produce separated harmonic echosignals along image lines which are intermediate the adjacent scanlines.Received scanlines R²⁻ and R₃₊ are summed to produce harmonic echosignals along image line L2 which is intermediate image lines L1 and L3.Received scanlines R⁴⁻ and R₅₊ are summed to produce harmonic echosignals along image line L4 which is intermediate image lines L3 and L5,and so on across the image field. It is seen that this furthercombination of adjacent scanlines produces a harmonic image with twicethe line density of conventional pulse inversion imaging, using the sametransmit pulse sequence as the conventional technique.

It will be appreciated that adjacent received scanlines R₁₊ and R⁴⁻could also be used to separate the harmonic echoes of image line L2since this pair of adjacent scanlines, like the R²⁻,R₃₊ pair, resultsfrom oppositely phased transmit pulses. However, since the R₁₊,R⁴⁻ pairis separated by the transmit-receive intervals of two other scanlines,2− and 3+ and their received echoes, the R₁₊,R⁴⁻ pair is moresusceptible to motion artifacts than is the time sequential R²⁻, R₃₊pair. Hence in the preferred embodiment time sequential adjacentscanlines are used to form the even-numbered intermediate image lines.

The received scanline processing arrangement of FIG. 5 may be used toform the image lines shown in FIG. 1. Echoes are received by thetransducer array 10 following each transmit scanline and coherent echosignals are steered and focused by a receive beamformer 12 to produce asequence of echo signals along the received scanline. Each receivedscanline is coupled to a line buffer 14 which delays each scanline bythe time interval of a transmit-receive cycle such that the previousreceive scanline and the current receive scanline are simultaneouslyapplied to a summer 20. The summer 20 will therefore sum echoes of thetwo scanlines on a corresponding depth (z) basis, producing separatedharmonic echo signals. When the sign of one of the signals beingcombined is changed by a sign change circuit 18, the arrangement of FIG.5 will produce separated linear signals, as indicated by theharmonic/linear control signal applied to the sign change circuit. Analternate way to achieve the same result is to replace the summer 20with a difference circuit (subtractor). The separated harmonic or linearsignals are then coupled to subsequent processing circuitry of theultrasound system where the echo signals are detected, processed, anddisplayed in the usual manner.

Since the processing system of FIG. 5 processes pairs of sequentiallyreceived scanlines, the summer 20 can produce the followingcombinations, depending upon the setting of the sign change circuit:

Harmonic Components:

L1=(R₁₊+R²⁻); L2=(R²⁻+R₃₊); L3=(R₃₊+R⁴⁻)

L4=(R⁴⁻+R₅₊);

Linear Components:

L1=(R₁₊−R²⁻); L2=(−R²⁻+R₃₊); L3=(R₃₊−R⁴⁻);

L4=(−R⁴⁻+R₅₊);

This operation is equivalent to convolving a spatial filter of the form[1 1] with the received data that is acquired by alternating thepolarity (or phase) of the transmit pulse. Harmonic components andlinear components are separated by inverting/noninverting the sign ofthe received data prior to the convolution.

In FIG. 2 the lateral spacing of the transmitted and received scanlinesis doubled in comparison with the FIG. 1 embodiment. In the same manneras FIG. 1, transmit scanlines 1+ and 2− yield received scanlines R₁₊ andR²⁻, respectively, transmit scanlines 3+ and 4− yield received scanlinesR₃₊ and R⁴⁻, and so forth along the array 10, but at a two elementspacing instead of a single element spacing. The received scanlines areprocessed by the processing system of FIG. 5 in the same manner asbefore, producing image lines L1, L2, L3, L4, . . . of harmonic echoinformation but, due to the doubled scanline spacing, the image linesare of the same line density as the conventional pulse inversiontechnique. Compare the image line spacing of FIG. 2 with the image linespacing of the odd-numbered image lines of FIG. 1. But since scanlinesare transmitted and received at twice the spacing as in FIG. 1, onlyhalf as many transmit-receive intervals are required and the image linesfor a full image frame at the conventional image line density areacquired in half the time. Thus, the frame rate of an image produced inaccordance with the scanning sequence of FIG. 2 is twice theconventional pulse inversion frame rate.

If the paired received scanlines (e.g., R₁₊,R²⁻ in FIG. 2) are offsetfrom each other rather than being co-aligned as shown in FIGS. 1 and 2,the combination of the two received scanlines will produce an image lineof harmonic echoes at a line location intermediate that of the receivedscanlines.

The present inventors have found that the scanning techniques of FIGS. 1and 2 can develop an artifact due to the line to line aperturevariation. In FIG. 2, for instance, it is seen that each pair ofspatially aligned receive scanlines such as R₁₊ and R²⁻ are spatiallyaligned with each other and with their respective transmit scanlines.That is, both the transmit and receive apertures for the scanlines whichare combined are commonly aligned and in spatial alignment with theresultant odd-numbered image line. But the even-numbered image lines areformed by combining scanlines from unaligned apertures. For example,received scanline R²⁻ and its transmit scanline are centered between thefirst and second elements e of the array 10 while received scanline R₃₊and its transmit scanline are centered between the third and fourthelements of the array. Thus, each image line across the image field isalternately formed from echoes from aligned and unaligned apertures.This alternation across the image can result in an annoying “picketfence” artifact in the image, particularly in the case of motion in theimage field.

One approach to reducing this picket fence artifact is to filter, oraverage, consecutively received scanlines. Filtered image lines withreduced artifacts may be produced by convolving a filter of the form [12 1] with the received scanline data acquired from alternate polaritytransmit pulses. As in the case of the arrangement of FIG. 5, eitherharmonic or linear components may be separated by selectively invertingor noninverting the sign of the received data prior to convolution. The[1 2 1] filter will produce the following image lines:

Harmonic Components:

L1=(R₁₊+2R²⁻+R₃₊); L2=(R²⁻+2R₃₊+R⁴⁻);

L3=(R₃₊+2R⁴⁻+R₅₊); L4=(R⁴⁻+2R₅₊+R⁶⁻); . . .

Linear Components:

L1=(R₁₊−2R²⁻+R₃₊); L2=(−R²⁻+2R₃₊−R⁴⁻);

L3=(R₃₊−2R⁴⁻+R₅₊); L4=(−R⁴⁻+2R₅₊−R⁶⁻); . . .

Consistent with the principle of pulse inversion, it is seen that eachof the separated components is composed of an equal contribution ofechoes from both positive and negative (opposite polarity) transmitpulses. For instance, harmonic image line L1 is composed of two samplesfrom positive transmit pulses (R₁₊ and R₃₊) and two samples fromnegative transmit pulses (2R²⁻).

An arrangement which implements the foregoing [1 2 1] filter is shown inFIG. 6. This arrangement is similar to that of FIG. 5, but includes asecond line buffer 14′ to twice-delay received scanlines and a secondsummer 20′ which produces the 2R_(xx) term for the [1 2 1] filterfunction. In this embodiment the sign change circuit 18′ functions bypassing received data without alteration when harmonic components arebeing produced, and by changing the sign of alternate received scanlineswhen linear components are being produced. From the form of the linearcomponents shown above, it is seen that the signs of the even-numberedreceived scanlines are inverted (R²⁻, R⁴⁻, etc.) and the signs of theodd-numbered received scanlines (R₁₊, R₃₊, R₅₊) are unchanged. Thisoperation of the sign changing circuit of changing the sign of alternatelines is also effective for the embodiment of FIG. 5.

Another scanning technique for high frame rate pulse inversion imagingis shown in FIG. 3. In this embodiment each received scanline isreceived at an aperture offset from the transmit aperture by one elementspacing. For instance, the echoes from transmit scanline 1+ are receivedat a received scanline R₁₊ aperture which is offset one element to theleft of its transmit aperture, and the echoes from transmit scanline 2−are received at a received scanline R²⁻ aperture which is offset oneelement to the right of its transmit aperture. Likewise, the echoes fromtransmit scanline 3+ are received at a received scanline R₃₊ aperturewhich is offset one element to the left of its transmit aperture, andthe echoes from transmit scanline 4− are received at a received scanlineR⁴⁻ aperture which is offset one element to the right of its transmitaperture, and so on. When image line L1 is formed from receivedscanlines R₁₊ and R²⁻, it is seen that the image line is in alignmentwith one aperture, the transmit aperture, but the other aperture, thereceive aperture, is split on either side of the position of the imageline. In like manner, while image line L2 is in alignment with oneaperture, in this case the receive apertures of scanlines R²⁻ and R₃₊,the transmit apertures of scanlines 2− and 3+ are split on either sideof the position of the image line. Thus there is a common characteristicacross the image field: each image line is in alignment with oneaperture (transmit or receive) and unaligned with the other (which issplit one element to either side of the image line.) While no image lineis in complete alignment with both apertures, the uniformity of theaperture nonalignment across the image field will reduce the artifactresulting from the alternating aperture characteristics of FIGS. 1 and2.

In digital ultrasound systems, received echoes are dynamically focusedand temporally sampled. Such a sampled data system exhibits certaincharacteristics which require specific processing to avoid imageartifacts. In particular, when the scanning technique of FIG. 3 isimplemented in a digital ultrasound system, interpolated samples onimage lines aligned with the transmit aperture are misaligned row by rowwith interpolated samples on image lines aligned with the receiveaperture.

FIGS. 3A-3C illustrate this problem. In FIG. 3A, transmit apertures TLand TR are aligned with an image line LT. The respective receiveapertures OL and OR are located to the left and to the right of thetransmit apertures. Echo data samples RL_(x) and RR_(x) are receivedfrom apertures OL and OR at uniformly spaced distances (or uniformlyspaced times) x in the image field. When these data samples RL_(x) andRR_(x) are combined to interpolate image line samples at the desiredintermediate locations 60, 62, 64 in line with the transmit aperture,the resultant interpolated samples are not uniformly spaced but arelocated on the intersections of the isochrons 66 of the respectivereceive apertures as shown by interpolated samples 61, 63, and 65.

FIG. 3B illustrates the spacing of the samples when the interpolatedimage line is aligned with the receive apertures. The transmit aperturesTL and TR are located to the left and right of the aligned receiveapertures OL and OR. Echo data samples RL_(x) and RR_(x), being receivedfrom aligned received apertures OL and OR, will result in uniformlyspaced image line samples 70, 72, 74. This drawing shows thatinterpolated samples from aligned receive apertures will remainuniformly spaced.

When the interpolated samples of FIGS. 3A and 3B are combined in animage field, it is seen that the spacing of the image line samplesvaries from one image line to the next across the image field, as shownby FIG. 3C. Image line samples aligned with the transmit aperturesexhibit one spacing, and image line samples aligned with the receiveapertures exhibit another spacing. These unregistered image samples willresult in a “shimmering” artifact, particularly in the near field of theresultant image where the misregistration is the most severe.

One way to remedy this shimmering artifact problem is to employ a signalresampling process. Axial resampling can recalculate sample values atthe desired locations along the image line using the values of theacquired, misregistered samples of the image line. The resamplingprocess can create its own artifact if only the misregistered imagelines are resampled, for this will alter the bandwidth from line toline. Such artifacts can be reduced by employing a double resamplingprocess on all image lines, computing intermediate values first, andthen final values at the desired sample locations on the lines, or byresampling all image lines to a sample alignment different from that ofboth types of received sample alignments. Since the pulse inversionprocess is a linear operation, the resampling process can be implementedbefore or after harmonic/linear separation.

The shimmering artifact can also be eliminated by processing thereceived scanlines using a spatial filter. Previous examples havedemonstrated the use of [1 1] and [1 2 1] filters. In the case of the [12 1] filter it is seen that

R₁₊+2R²⁻+R₃₊=(R₁₊+R²⁻)+(R²⁻+R₃₊)

This shows that the effect of applying a [1 2 1] filter is equivalent toaveraging the transmit aligned pixel with the receive aligned pixel toreduce the image artifact. But the [1 2 1] filter will only reduce, noteliminate, the artifact since the transmit and receive pixel aperturesare not perfectly aligned.

A higher order [1 3 3 1] filter will effectively eliminate the artifact,however, as illustrated by FIG. 3D. In this drawing the samples 70-79show the alternate alignment of image line pixel data as a result of [11] filtering, as in FIG. 3C. The odd-numbered image lines are alignedwith the transmit scanlines and the even-numbered image lines arealigned with the received scanlines and the scanlines are equallylaterally spaced as a result of the scanning format of FIG. 3. Toregister this image data laterally, each pair of consecutivetransmit-aligned scanlines is interpolated to produce intermediate pixeldata aligned with the receive-aligned scanlines, and each pair ofconsecutive receive-aligned scanlines is interpolated to produceintermediate pixel data aligned with the transmit-aligned scanlines.This intermediate pixel data is then averaged with the axially adjacentsample data to produce the desired image data.

As an example, the data on lines L1, L2 and L3 is initially of the form:

L1: (R₁₊+R²⁻) L2: (R²⁻+R₃₊) L3: (R₃₊+R⁴⁻)

From the two adjacent scanlines aligned with the transmit beams, L1 andL3, intermediate interpolated scanline data 80,81 is produced:

(L1+L3)/2: 0.5((R₁₊+R²⁻)+(R₃₊+R⁴⁻))

This intermediate interpolated data is axially interpolated with theuninterpolated data on the receive aligned scanline L2 to produce thedesired aligned interpolated pixels 90,91:

(L1+L3)/4+L2/2=L1/4+L3/4+L2/2: 0.25(R₁₊+3R²⁻+3R₃₊+R⁴⁻)

In the same manner, intermediate interpolated scanline data 82,83 isproduced from adjacent scanlines aligned with the received scanlines:

(L2+L4)/2: 0.5((R²⁻+R₃₊)+(R⁴⁻+R₅₊))

This intermediate interpolated data is axially interpolated with theuninterpolated data on the transmit aligned scanline L3 to producedesired aligned interpolated pixels 92,93:

(L2+L4)/4+L3/2=L2/4+L4/4+L3/2: 0.25(R²⁻+3R₃₊+3R⁴⁻+R₅₊)

Neglecting the scaling factor of 0.25, the aligned pixels are of theform

L2: R₁₊+3R²⁻+3R₃₊+R⁴⁻ L3: R²⁻+3R₃₊+3R⁴⁻+R₅

This is effectively equivalent to processing the scanline data with a [13 3 1] filter. The preceding example describes the processing toseparate harmonic image line data. Linear image line data can beobtained by inverting the sign of the received data acquired in responseto the transmit pulses of inverted (negative) sign or polarity. Theseparated linear components will thus be of the form

L2: R₁₊3R²⁻+3R₃₊−R⁴⁻ L3: −R²⁻+3R₃₊−3R⁴⁻+R₅

Use of this processing technique can eliminate the shimmering artifactas a result of realignment of the image pixels from image line to imageline. Although the image pixels are not sampled perfectly uniformly intheory, the sampling error is negligibly small when the image lines areprocessed with a [1 3 3 1] or higher order filter.

FIG. 7 illustrates a processing system which simultaneously separatesboth linear and harmonic echo signal components, then blends themtogether in a single image as a function of depth. Such a blended imagecan take advantage of the low nearfield clutter performance which ispossible with harmonic components, and the better depth penetration oflinear components. In FIG. 7 the summer 20 additively combinessequential scanlines received from oppositely phased transmit signals toproduce separated harmonic signal components as in the case of theembodiment of FIG. 5. A subtractor 24 takes the difference of sequentialscanlines received from oppositely phased transmit signals to produceseparated linear (fundamental) signals. The signals from the summer 20and subtractor 24 can if desired be separately processed and displayedas separate or overlaid harmonic and fundamental images. In thisembodiment the respective harmonic and fundamental signals aremultiplied by weighting functions by multipliers 22 and 26. The harmonicsignal components are weighted by a depth-variable weighting factork_(h)(z). The linear signal components are also weighted by adepth-variable weighting factor kl(z). In a preferred embodiment theweighting factors vary in an inverse relationship, with harmoniccomponents more heavily weighted in the near field and linear componentsmore heavily weighted in the far field.

The weighted signal components are combined by a summer 30, thenforwarded for detection, image processing, and display.

It will be appreciated that other factors can be used to control thevariability of the weighting factors such as other spatial dimensions ortime.

Variable blending can take advantage of the different characteristics oflinear and harmonic signals in different imaging applications.

A multiline technique producing an even greater frame rate of display isshown in FIG. 4. In this embodiment only a single transmit scanline isproduced at a two element spacing. The polarity or phasing of thetransmit pulses alternates from one transmit scanline to the next. Theacoustic field of each transmitted scanline is broad enough to encompasstwo receive scanlines which are simultaneously received, as shown inU.S. Pat. No. 4,644,795. Thus, “multiline” reception is employed formultiple scanlines following each transmit wave.

In the illustrated embodiment transmit scanline 1+ results in thereception of a receive scanline to the right and left of the center ofthe transmit aperture, R_(1L+) and R_(1R+). Similarly, transmit scanline2− results in the reception of receive scanlines R_(2L−) and R_(2R−),and transmit scanline 3+ results in the reception of receive scanlinesR_(3L+) and R_(3R+), and so on. In this embodiment the receivedscanlines are spaced one element to the left and right of the transmitscanline so that successive received scanlines are in alignment,however, this is not required; the technique is applicable even when thereceived scanlines do not overlap, although care must be taken to avoidartifacts from spatial aliasing when greater scanline spacing isemployed.

Successively received scanlines are then combined to produce separatedharmonic (or linear) signals along the image lines depicted at thebottom of FIG. 4. Image line L1 is formed by combining receivedscanlines R_(1L+) and R_(2L−), which are derived from oppositely phasedtransmit signals. Image line L2 is formed by combining receivedscanlines R_(1R+) and R_(2L−), which are likewise derived fromoppositely phased transmit signals. Image line L3 can be formed bycombining R_(1R+) and R_(2R−), or by combining R_(2L−) and R_(3L+). Likethe embodiment of FIG. 3, each image line is aligned with one of thetransmit or receive apertures, and unaligned with respect to the other,which is split on either side of the image line. These image lines aresubject to the same artifacts as the FIG. 3 technique, and thus benefitfrom higher order filtering. Using the [1 3 3 1] filter, image line L2is formed by combining received scanlinesR_(1L+)+3R_(1R+)+3R_(2L−)+R_(2R−) which are derived from consecutive,oppositely phased transmit signals. Image line L3 is formed by combiningreceived scanlines R_(1R+)+3R_(2L−)+3R_(2R−)+R_(3L+), which are likewisederived from consecutive, oppositely phased transmit signals. Image lineL4 can be formed by combining R_(2L−)+3R_(2R−)+3R_(3L+)+R_(3R+). Likethe embodiment of FIG. 3, each image line is aligned with one of thetransmit or receive apertures, and unaligned with respect to the other,which is split on either side of the image line. Thus, this filteringtechnique has the same beneficial artifact performance as in theembodiment of FIG. 3.

An ultrasound receive signal processor for performing the scanningtechnique of FIG. 4 is shown in FIG. 8. In this system the elements ofthe transducer array 10 are coupled to individual channels of atransmitter 16, which provide individually timed transmit signals toeach element to steer and focus the transmit scanline beam as desired.The transducer elements are also coupled in parallel to the inputs oftwo receive beamformers, beamformer 12L and beamformer 12R, preferablyby multiplexing which enables the inputs to the beamformers to bechanged so that they may be operated as two multiline beamformers or asone single-line beamformer. The received scanlines produced by the twobeamformers are coupled to the inputs of line buffers 14 and to theinputs of summers 42, 44, and 46. The summers combine sequentialscanlines from opposite polarity transmit pulses and produce image linesof separated harmonic echo components. Separated linear signalcomponents can be obtained by the use of sign change circuits at theoutput of each beamformer (not shown) to change the sign of signalsreceived from alternate transmit pulses. A filter and line sequencer 50receives image line data from the three summers, buffers the data asrequired, and transmits image line data to the detection, processing anddisplay circuitry of the ultrasound system in a desired image linesequence. Alternatively, the filter and line sequencer can comprise amultiple entry frame store which stores multiple selected image linesfor subsequent processing and display.

In operation, beamformer 12L will sequentially produce scanlinesR_(1L+), R_(2L−), R_(3L+), R_(4L−), and so on in response to thetransmit scanline sequence of FIG. 4. The beamformer 12R willconcurrently produce scanlines R_(1R+), R_(2R−), R_(3R+), R_(4R−), andso on. These sequences result in the production of the followingcombinations at the outputs of the summers:

(R_(1L+)+R_(2L−)) (R_(2L−)+R_(1R+)) (R_(1R+)+R_(2R−)) (R_(2R−)+R_(3L+))(R_(3L+)+R_(4L−)) (R_(4L−)+R_(3R+)) (R_(3R+)+R_(4R−)) . . .

where this sequence of image lines is produced by summer 42, summer 44,summer 46, summer 44, summer 42, summer 44, summer 46, and so on.Sequencing the summer outputs in this order will produce the image linesequence L1, L2, L3, L4, L5, and so on as shown at the bottom of FIG. 4.

Since the scanning technique of FIG. 4 will benefit by the samefiltering techniques as the previous embodiment, in a preferredembodiment of FIG. 8, the filter and line sequencer 50 processes thescanline data with a [1 3 3 1] filter in the same manner as the previousembodiment. This will lead to the production of harmonic signalcomponents of the form:

L2: (R_(1L+)+3R_(1R+)+3R_(2L−)+R_(2R−))

L3: (R_(1R+)+3R_(2L−)+3R_(2R−)+R_(3L+))

L4: (R_(2L−)+3R_(2R−)+3R_(3L+)+R_(3R+))

L5: (R_(2R−)+3R_(3L+)+3R_(3R+)+R_(4L−))

L6: (R_(3L+)+3R_(3R+)+3R_(4L−)+R_(4R−))

By inverting the sign of the received data acquired in response to thetransmit pulses of inverted (negative) sign or polarity, the [1 3 3 1]filter will produce linear signal components of the form:

L2: (R_(1L+)+3R_(1R+)−3R_(2L−)−R_(2R−))

L3: (R_(1R+)−3R_(2L−)−3R_(2R−)+R_(3L+))

L4: (−R_(2L−)−3R_(2R−)+3R_(3L+)+R_(3R+))

L5: (−R_(2R−)+3R_(3L+)+3R_(3R+)−R_(4L−))

L6: (R_(3L+)+3R_(3R+)−3R_(4L−)−R_(4R−))

In accordance with the principles of the present invention, the scanningtechnique of FIG. 4 is applied to the interpolation of multilinereceived signals to cure a defect of prior art arrangements. Thesimplest conventional multiline sequence is to receive two scanlines forevery transmit pulse, one scanline on either side of the center of thetransmit beam. One prior art interpolation technique forms one imageline by averaging the two received scanlines, and another image line byaveraging adjacent scanlines from two consecutive transmit pulses, thatis, the scanline to the left of one transmit beam is averaged with thescanline to the right of the neighboring transmit beam. Receivedscanlines across the image field are averaged in this manner to developan image of interpolated scanlines. This interpolation technique issusceptible to an alternating motion artifact similar to that describedabove, because the first pair of scanlines (and every odd-numbered pair)which are averaged are concurrently received and the second pair ofscanlines (and every even-numbered pair) which are averaged aresequentially received. If there is motion in the image field, theconcurrently received scanlines will be equally affected because theyare produced by a single transmit pulse. The sequentially receivedscanlines will be differently affected by motion because they aredeveloped by temporally different transmit pulses and each scanline willreflect the position of moving materials as of the moment they areproduced. Thus, odd-numbered lines will not have a motion artifact andeven-numbered lines will, creating a scintillation-type artifact whenthere is motion in the image plane.

In accordance with the principles of the present invention, this motionartifact of multiline interpolation is avoided as shown in FIG. 9. Inthis drawing a sequence of transmit pulses T₁, T₃, T₅, . . . istransmitted across the image field. A pair of scanlines are received inresponse to each transmit pulse. For instance, scanlines R₁₀ and R₁₂ arereceived in response to transmit pulse T₁, where the first subscriptrefers to the number of the transmit pulse which produced the scanline,and the second subscript refers to the relative position of the receivedscanline across the image field. The second transmit pulse T₃ results inthe reception of scanlines R₃₂ and R₃₄, and so on.

The prior art technique for forming interpolated image lines from thesescanlines is to average R₁₀ and R₁₂ to form image line L1; R₁₂ and R₃₂to form image line L2; and so on across the image field. But L1 isformed from temporally identical received scanlines and L2 is formedfrom temporally different received scanlines, giving rise to the motionartifact. The technique of the present invention overcomes this problemby consistently forming image lines from temporally different receivedscanlines. That is, L1 is formed by interpolating scanlines R₁₀ and R₃₂,image line L2 is formed by interpolating scanlines R₁₂ and R₃₂, imageline L3 is formed by interpolating scanlines R₃₂ and R₅₄, and so on asshown by the ovals 1, 2 and 3 encompassing these scanline designators inFIG. 9.

This results in a uniform characteristic of the interpolated image linesacross the image field, since each image line is formed from twotemporally different scanlines. This method of interpolation iscontinued across the image field as the encompassing ovals 4, 5, and 6in FIG. 9 indicate.

The present inventors have noted that, while the above two-lineinterpolation technique reduces the motion scintillation artifact, anerror is introduced by the variation in the locations of the transmitand receive apertures from line to line across the image field. Hence,the present inventors prefer the use of al three-line lateral filter ofthe form (R_(x)+2R_(y)+R_(z)) for use as the line interpolator for FIG.9. This set of filter coefficients advantageously weights two scanlinesacquired on one side of a transmit beam with a double-weighting of ascanline acquired on the other side. This filter will form an image lineL1.5 by interpolating scanlines R₁₀, 2R₁₂ and R₃₂; image line L2.5 isformed by interpolating scanlines R₁₂, 2R₃₂, and R₃₄; image line L3.5 isformed by interpolating scanlines R₃₂, 2R₃₄ and R₅₄, and so on. Each ofthese interpolated lines is intermediate the integer lines shown in FIG.9. This filter form is also effective for performing high frame ratepulse inversion harmonic separation. Referring to FIG. 4, for instance,it may be seen that a lateral filter of this form will sequentiallyproduce

(R_(1L+)+2R_(2L−)+R_(1R+)) (R_(2L−)+2R_(1R+)+R_(2R−))(R_(1R+)+2R_(2R−)+R_(3L+))

This sequence is seen to combine two weights of scanlines received frompositive (or negative) polarity transmit pulses with two weights ofscanlines received from negative (or positive) polarity transmit pulses,thereby providing complete harmonic separation.

The image line interpolation technique of FIG. 9 can be carried out bythe multiline receiver and interpolation arrangement of FIG. 10. In thissystem the elements e of the array transducer 10 are individually drivenby a transmitter 136 to steer a focused transmit beam from the desiredpoint along the transducer array in the desired direction in the imagefield. Conductors or, preferably, multiplexers couple echo signalsreceived by the elements e to a multiline beamformer 132,134, asindicated by the arrow 130. The multiline beamformer 132,134 may be twoseparate beamformers, or separately controllable and separately summingpartitions of a single beamformer. Each multiline beamformer partitionproduces a received scanline, one to the left and one to the right ofthe transmit beam transmitted under control of transmitter 136. Eachpair of concurrently received scanlines is stored temporarily in a linememory 138 which acts as a buffer, and forwarded at the appropriate timewith subsequently received scanlines to line interpolator 140. The lineinterpolator forms an image line by interpolating pairs of scanlines asshown in FIG. 9, or preferably implements a three-line lateral filter ofthe form (R_(x)+2R_(y)+R_(z)). The interpolated image lines are coupledto detection and signal processing circuitry 142 and a scan converter146 for processing and display of an image on a display 150. It is seenthat the apparatus of FIG. 10 when operated in accordance with thetechnique of FIG. 9 can form high frame rate interpolated multilineimages of virtually every sort of echo signal. This apparatus andtechnique will find use in B mode, Doppler and harmonic imaging.

What is claimed is:
 1. A method for producing an ultrasonic image of interpolated image lines with reduced susceptance to motion artifacts including at least one level of interpolation comprising the steps of: transmitting a plurality of laterally spaced ultrasonic beams along transmit beam locations in an image field; receiving a plurality of laterally spaced receive beams which are located laterally on opposite sides of the transmit beam location in response to each transmitted beam; and producing interpolated image lines from said receive beams, which comprise interpolated image lines, at least some of which are formed from temporally different receive beams located on opposite sides of a transmit beam location; and detecting and displaying said interpolated image lines, wherein each of the interpolated image lines formed at each level of interpolation are formed from a plurality of temporally different lines.
 2. The method of claim 1, wherein the step of transmitting transmits a sequence of laterally spaced beams of the form T₁, T₃, T₅, T₇, wherein the subscripts refer to the lateral spatial sequence of the transmit beams.
 3. The method of claim 2, wherein the step of receiving receives pairs of scanlines of the form (R₁₀,R₁₂) in response to transmit beam T₁, (R₃₂,R₃₄) in response to transmit beam T₃, (R₅₄,R₅₆) in response to transmit beam T₅, (R₇₆,R₇₈,) in response to transmit beam T₇, wherein the first subscript refers to the number of the transmit pulse which produced the scanline, and the second subscript refers to the relative lateral position of the received scanline across the image field.
 4. The method of claim 3, wherein the step of producing interpolated image lines produces lines of the form (R₁₀+R₃₂), (R₁₂+R₃₂), (R₃₂+R₅₄), (R₃₄+R₅₄).
 5. The method of claim 3, wherein the step of producing interpolated image lines comprises the step of laterally filtering two or more adjacent received scanlines.
 6. The method of claim 5, wherein the step of laterally filtering comprises using a filter function of the form [1 2 1] to produce image lines of the form (R_(x)+2R_(y)+R_(z)).
 7. The method of claim 6, wherein the step of laterally filtering produces image lines of the form (R₁₀+2R₁₂+R₃₂), (R₁₂+2R₃₂+R₃₄), (R₃₂+2R₃₄+R₅₄).
 8. An ultrasonic diagnostic imaging system which produces an ultrasonic image of interpolated image lines with reduced susceptance to motion artifacts comprising: an array transducer; a beamformer, coupled to said array transducer, which controls said array transducer to transmit a sequence of laterally spaced ultrasonic beams along transmit beam locations in an image field, and which forms laterally spaced scanlines on opposite sides of the transmit beam location in response to each transmitted beam; and an interpolator, coupled to said beamformer and responsive to said scanlines to perform at least one level of interpolation of said scanlines, which produces interpolated image lines, at least some of which are formed from temporally different receive beams located on opposite sides of a transmit beam location; and an image processor which detects and displays said interpolated image lines, wherein the interpolated image lines formed at each level of interpolation are formed from a plurality of temporally different lines.
 9. The ultrasonic diagnostic imaging system of claim 8, wherein said beamformer includes a receive beamformer which forms two scanlines in response to each transmitted beam on opposite sides of each transmit beam location.
 10. The ultrasonic diagnostic imaging system of claim 9, wherein said interpolator comprises a lateral filter.
 11. The ultrasonic diagnostic imaging system of claim 10, wherein said lateral filter performs a filter function of the form (R_(x)+R_(y))/2 to produce successive image lines, wherein R_(x) and R_(y) are scanlines produced by said receive beamformer in response to different transmitted beams.
 12. The ultrasonic diagnostic imaging system of claim 10, wherein said lateral filter is a [1 2 1] filter which performs a filter function of the form (R_(x)+2R_(y)+R_(z)) to produce successive image lines, wherein two of R_(x), R_(y) and R_(z) are scanlines produced by said receive beamformer in response to different transmitted beams.
 13. The ultrasonic diagnostic imaging system of claim 8, wherein said interpolator further acts to produce interpolated image lines formed from temporally different receive beams located between successive transmit beam locations.
 14. The ultrasonic diagnostic imaging system of claim 8, wherein the interpolator acts to produce interpolated image lines with enhanced harmonic signal content through interpolation of temporally different receive beams received in response to differently phased transmit beams.
 15. The method of claim 1, wherein producing interpolated image lines further comprises forming interpolated image lines from temporally different receive beams located between successive transmit beam locations.
 16. The method of claim 1, wherein the interpolated image lines are further formed from receive beams received in response to differently phased transmit beams, wherein each of the interpolated image lines contain harmonic signal information enhanced in comparison to fundamental signal information. 